Aneuploidy is defined as an alteration in chromosome number that is not a multiple of the haploid complement. Such karyotypic changes are associated with human disease. Aneuploidy is the leading cause of miscarriages and mental retardation in humans. It is also a hallmark of cancer: more than 90% of all solid human tumors are aneuploid. In order to shed light on the relationship between aneuploidy and cancer we must understand how aneuploidy affects the physiology of normal cells. We have developed several cell models of aneuploidy in yeast and mouse to address this question. Our studies in yeast revealed a set of phenotypes shared among many different aneuploidies, which we call the aneuploidy-associated stresses. They include a transcriptional stress response, a cell proliferation defect, increased need for energy, genome instability and proteotoxic stress. Importantly, our studies in mammalian cells revealed that these aneuploidy-associated stresses are conserved across eukaryotes. This discovery indicates that our studies in yeast will provide critical insights into how the aneuploid condition affects mammalian cell physiology and tumorigenesis. Here we propose to study two aneuploidy-associated phenotypes and to investigate how they are connected. We will determine the molecular basis for aneuploidy-induced proteotoxicity and how it contributes to the proliferation defects of aneuploid cells. In addition, we will explore the exciting possibility of a link between aneuploidy and aging. In Specific Aim 1 we will determine how proteomic imbalances caused by aneuploidy lead to proteotoxic stress. Specifically, we will distinguish between the possibilities that protein qualit control pathways are defective or overwhelmed in aneuploid cells. In Specific Aim 2 we will determine how proteotoxic stress impacts cell proliferation, especially the G1 - S phase transition that is found to be impaired in many different aneuploid yeast strains. Finally, in Specific Aim 3 we will investigate whether aneuploidy is a cause of aging. Our preliminary data raise this exciting possibility. We will now determine whether aneuploidy increases with replicative age and whether specific aneuploidies cause the phenotypes that are so characteristic of aging. Cancers are highly aneuploid and under profound proteotoxic stress. Determining which protein quality control pathways are vulnerable in cells with an altered karyotype is thus highly relevant to understanding the physiological state of cancer cells. Given that the cellular phenotypes of aging are conserved across eukaryotes our investigation of the relationship between aneuploidy and aging is likely to also have important implications for cellular aging in humans.